Opioids comprise one of the most effective class of drugs for the clinical management of pain. Electroencephalographic data indicate that opioids significantly inhibit the rapid eye movement (REM) phase of sleep. This opioid-induced REM sleep inhibition has come to be understood as a major unwanted side effect of opioids. Furthermore, numerous studies have shown that when opioid therapy is discontinued there is a rebound increase in REM sleep that is associated with cardiopulmonary complications. Three lines of evidence emphasizes the health relatedness of opioid-induced REM sleep inhibition. First, since opioids are commonly used in the postoperative setting, a better understanding of opioid-induced cardiopulmonary complications is relevant for anesthesia and peri-operative medicine. Second. opioid therapy also is common for chronic pain management, and sleep disruption is the number one complaint of individuals suffering from chronic pain. Third, related to sleep disorders and pulmonary medicine is the fact that the periodic cessation of breathing (apnea) is most severe during REM phase of sleep cycle. the brain mechanisms causing opioid-induced REM sleep inhibition are poorly understood. The long-term goal of the four specific aims is to specify the cellular and molecular mechanisms through which opioids cause REM sleep inhibition. Brain stem cholinergic systems are known to play a major role in REM sleep generation, and the proposed experiments will study the effects of opioids on cholinergic neurotransmission. Aim 1 will test the hypothesis that microinjection of opioid agonists into the cholinergic laterodorsal and pedunculopontine tegmental (LDT/PPT) nuclei will cause REM sleep inhibition and respiratory depression. Opioid receptors are linked to G proteins, and Aim 2 will test the hypothesis that in the medial pontine reticular formation (mPRF) the mu receptor/adenylyl cyclase/cAMP cascade modulates opioid-induced REM sleep inhibition. Aim 3 will use microdialysis and HPLC to test the hypothesis that acetylcholine (ACh) levels in the mPRF are decreased by administering opioid agonists into the LDT/PPT. Inhibition of nitric oxide synthase (NOS) decreases ACh levels in the mPRF, and Aim 4 will test the hypothesis that mPRF administration of NOS inhibitors will potentiate the REM sleep inhibition and respiratory depression previously shown to be caused by mPRF opioids. These aims are conceptually unified, and related to the long-term goal, by their ability to specify brain stem cholinergic neurons and mechanisms causally contributing to opioid-induced inhibition of REM sleep.